1. Field of the Invention
The present invention relates to a circuit substrate or particularly relates to a thermally conductive substrate employed for packaging power electronics, a manufacturing method therefor and a power module incorporating a thermally conductive substrate.
2. Description of the Related Art
In recent years, following improvement in the performance of electronic equipment and demand for miniaturization, it is desired to increase the density of a semiconductor and to improve the function thereof. Due to this, circuit substrates for packaging semiconductors are also desired to be small in size and high in density. As a result, design in which radiation of the circuit substrate is taken into consideration becomes important.
As circuit substrates exhibiting good radiating property, various types of circuit substrates have been developed conventionally. It is not, however, easy to maintain a circuit substrate to have good radiating property while holding down the price thereof.
As thermally conductive substrates to solve such disadvantages, there is conventionally known one disclosed by U.S. Pat. No. 6,060,150. The thermally conductive substrate disclosed therein is constituted by forming an insulator sheet having a sufficient thermally conductive filler to increase radiating property, lead frame and a radiation plate integrally with one another.
This substrate is manufactured as follows. A film is formed by mixing thermosetting resin having flexibility in an unhardened state with a thermally conductive filler (or an inorganic filler) to thereby produce an insulator sheet filled with the inorganic filler at high concentration for a thermally conductive substrate. Then, the insulator sheet, the lead frame and the radiation plate are built up with the insulator sheet put between the lead frame and the radiation sheet and the resultant buildup layers are heated and pressurized. As a result, the insulator sheet flows in the surfaces of the lead frame and hardened, thereby integrating the lead frame and the insulator sheet with each other. Further, the radiation plate adheres to the surface of the insulator sheet opposite to the surface to which the lead frame adheres.
Here, for the purpose of stabilizing the shapes of the lead frame and facilitating a processing for integrating the lead frame with the insulator sheet, all the end portions of the lead frame are coupled to a frame-shaped common outer peripheral portion. Due to this, the outside residues and outer peripheral portion of the lead frame on the thermally conductive substrate thus manufactured are removed except for the necessary parts of inside of the lead frame.
FIG. 13 shows the thermally conductive substrate thus manufactured. In case of the thermally conductive substrate shown in FIG. 13, the lead frame 800 protruding to the side surfaces of the thennally conductive substrate are bent perpendicularly (or in a direction orthogonal to substrate surface) so as to use the tip ends of the lead frame 800 as external lead electrodes.
The thermally conductive substrate stated above has disadvantages in that discharge tends to occur between the lead frame 800 and the radiation plate 802, thereby disadvantageously causing the damage of the substrate. The reason is as follows. To maintain the good thermal conductivity of the thermally conductive substrate, it is necessary to thin the insulator sheet 801. If so, however, the lead frame 800 and the radiation plate 802 become excessively adjacent each other and the creeping distance between the radiation plate 802 and the lead frame 800 cannot be sufficiently secured.
To prevent such a discharge phenomenon, it is proposed to secure a large creeping distance by locating the bent portions 800a of the lead frame 800 at a positions slightly inside of the end faces of the insulator sheet 801 in the sheet plane direction.
If so, however, the lead frame 800 is bent inside compared with the end faces of the substrate, with the result that a region on the thermally conductive substrate in which components can be actually mounted becomes small relatively to the substrate size.
Further, to bend the lead frame 800 toward the inside of the sheet plane of the insulator sheet 801 relatively to the end faces of the sheet 801, the lead frame 800 is required to be peeled off from the insulator sheet 801. This may probably damage the thermally conductive substrate.
To avoid such damage, it is proposed take the following measures. By providing steps on the insulator sheet 801 on portions (i.e., sheet end portions) of the sheet 801 on which the lead frame 800 is pulled out, the step portions of the lead frame 800 is exposed. By doing so, it is possible to prevent the thermally conductive substrate from being damaged without the need to peel off the bent portions 800a of the lead frame 800 from the insulator sheet 801.
To provide such steps on the insulator sheet 801, however, there is no avoiding making the shape of a metallic mold used to manufacture the substrate complex, which hampers cost reduction.